Holography using multiple diffused object illumination beams

ABSTRACT

A hologram recording apparatus comprises a diffraction grating which serves to convert an object light beam obtained from a single light source into a plurality of beams, and a diffuser which is arranged in the path of the multiple light, whereby an object is illuminated by light from said diffuser.

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Kanazawa et al. 1 Jan. 1, 1974 HOLOGRAPHY USING MULTIPLE 3.640.5992/1972 Van Lighten 35013.5

DIFFUSED OBJECT [LLUMINATION 3,659,914 5/1972 Brooks 350/345 BEAMS3,650,595 3/l972 Gerritsen et al. 350/35 3,490,827 l/l970 Van Lighten etal 350/35 [75] Inventors: Yasunori Kanazawa; Yoshizumi Eto,

both of Hachioji, Japan Primary Examiner-Ronald J. Stern [73] Assignee:Hitachi, Ltd., Tokyo, Japan Atmmey pau| Craig Jr et a1. 22 Filed: Oct.18, 1972 [21] App]. No.: 298,568

[57] ABSTRACT [30] Fore'gn Application Priority Data A hologramrecording apparatus comprises a diffracoct. 18, Japan v {ion gratingerves to convert an objecx [52] us Cl 350/3 5 350/162 R beam obtainedfrom a single light source into a plural- [51] InL'CI b 27/00 ity ofbeams, and a diffuser which is arranged in the [58] Field B 5 162 SFpath of the multiple light, whereby an object is illumi- 55 nated bylight from said diffuser.

[56] References Cited 6 Claims 3 Drawing Film" UNITED STATES PATENTS3,677,616 7/1972 Lewis 350/35 l/f/ I PATENIEII I 74 I PRIOR ART FIG.

FIG. 2 PRIOR ART FIG. 3

HOLOGRAPIIY USING MULTIPLE DIFFUSED OBJECT ILLUMINATION BEAMS BACKGROUNDOF THE INVENTION The present invention relates to a hologram recordingapparatus, and more particularly to an apparatus which, for recordingimages by the use of holography, enhances the signal-to-noise ratiowithout spoiling redundancy.

With respect to image recording with holography, several proposals havehertofore been made. However, there is not yet provided a system whichis high in redundancy and which attains such good picture quality thatthe signal-to-noise ratio can sufficiently endure practical use. Thehigh redundancy is necesary to prevent a lowering of the quality ofreconstructed images due to, e.g., a flaw in the recording medium and isgenerally obtained by illuminating an object with light diffused bymeans of a light diffuser. The light diffuser is formed of an opticallyrough surface, so that so-called speckle noise is inevitably generated.Therefore, this has remarkably degraded the signal-to-noise ratio of areconstructed image. On the other hand, in recording a digital pattern,utilization of a random phase shifter has been suggested as a means toobtain high redundancy. The suggestion, however, is not yet confirmedwhether it can also be an effective means where general pictures areused as the object. As another means to obtain high redundancy a methodhas also been suggested in which a plurality of unit holograms made fromphotographs of an identical object are combined into one hologram, andreconstructed images of the respective unit holograms are superposed atreconstruction, whereby noise components are cancelled by utilizing thecorrelation of the reconstruction image as required. With this method,however, the quality of picture as can be put into practical use is notobtained in the present state of the art.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a hologram recording apparatus by which a reconstructed image ofhologram having a good signal-to-noise ratio is obtainable withoutdeteriorating redundancy.

In order to accomplish such an object, the present invention makes ahologram using a diffraction grating which converts an object beamemergent from a single light source into a plurality of light beams, anda diffuser on which the beams from the diffraction grating areilluminated.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are schematic diagramsfor explaining the principle of the present invention; and

FIG. 3 is a schematic presentation of an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram forexplaining the function of a diffraction grating for use in a hologramcomposing apparatus according to the present invention.

Referring to the figure, numeral 1 designates a lens, 2 a diffractiongrating arranged at the front focal plane of the lens 1, 3 parallellight fluxes from a lightsource (not shown), and 4 a screen arranged atthe back focal plane of the lens 1.

In the case where the diffraction grating 2 is illuminated by theparallel light fluxes 3 with such a construction, a Fraunhoferdiffraction pattern of the diffraction grating 2 is formed on the screen4.

As is well known, it is the Fourier transform image of the grating 2,and provides a spacial frequency distribution of the grating 2. That isto say, light spots from light having been rectilinearly propagatedthrough the grating 2 (the so-called O-th order light beam) and lightspots of higher order terms determined by the spacial construction ofthe grating 2 appear on the screen 4. The number of light spots can befreely selected through the pitch of the grating 2 and the area ratiobetween the light blcoking or intercepting parts and the lighttransmitting parts thereof.

Now, the screen 4 in FIG. I is removed. As shown in FIG. 2, a lens 5 isarranged on the optical axis of the lens 1, so as to have a common focalpoint with the lens I. A group of light spots are produced in such a waythat the grating 2 undergoes a Fourier-transformation by the lens I, andare further Fourier-transformed by the lens 5, and are converted intoparallel plane waves again.

On the other hand, in the case where a diffuser, such as a ground glassplate scattering light at random, is inserted into the optical path ofcoherent light such as laser light, speckle noises arise at an imageplane thereof. It is also known that, by virtue of its randomness, thediffuser is useful as means by which input information can be recordedwith redundancy imparted thereto. During high-redundancy recordingemploying the diffuser, the speckle noises are unavoidably generated, toconspicuously lower the signal-to-noise ratio of pictures. Thedistribution of the speckle noises corresponds to the distribution ofdirections in which the light is scattered. If the position of thediffuser is moved within the optical path, the distributed state of thenoises is also changed. Accordingly, if diffusers differing in thedistribution of scattering directions from one another are arranged forthe respective light fluxes of the light spots occurring between thelens I and the lens 5 in FIG. 2, a light beam after passage through thelens 5 becomes a beam with the light rays of the distri' butions of thespeckle noises superposed on one another. If, in place of such aplurality of diffusers, a single diffuser extending over all the lightspots is used, the same result can be obtained, since the distributedstate of light on the diffuser varies in dependence on the location.

Thus, the use of the diffuser leads to the same result as that ofsuperposition of the same N pictures differing in the noisedistrubution. Therefore, in conformity with the idea of so-calledsynchronized addition, the noise component increases in proportion toVN, whereas the signal component increases in proportion to N. Thesignal-to-noise ratio is accordingly improved at a rate of \/N.

FIG. 3 is a diagram showing the construction of an embodiment of ahologram making apparatus according to the present invention andincluding a diffraction grating and a diffuser. Referring to the figure,numeral 6 indicates a light source such as laser, 7 a collimator lenssystem, 8 diffraction grating, 9 a lens, 10 a diffuser, 11 a lens, 12 anobject, 13 a lens, and 14 a screen. Numeral 15 represents a beamsplitter which is arranged between the light source 6 and the collimatorlens system 7, 16 a reflector which reflects a light beam from the beamsplitter l5, 17 a beam splitter, and 18 and 19 reflectors.

In such an arrangement, coherent light emerging from the light source 6is split into an object light beam and a reference light beam by thebeam splitter 15. The object light beam is converted by the collimatorlens system 7 into parallel plane waves with the light beam diameterexpanded.

The diffraction grating 8 located in the light path of the parallelplane waves has its diffraction pattern produced by the lens 9. Thus,the light beam having emerged from the single light source is convertedinto a plurality of light beams. The diffuser 10 is arranged, as shownin the figure, for the plurality of light beams obtained in this way.Then, the light sources include speckle noises of respectively differentdistributions.

Accordingly, where the light fluxes are again made parallel light raysby the lens 11 to illuminate the object 12, the speckle noises areaveraged on the plane of the object 12 for the reason stated above, andthe object is illuminated by the light with only slight noisecomponents. in order to obtain a Fourier transform hologram, an image ofthe object 12 is subjected to Fourier transform by the lens 13, and theimages is focused on the photographic emulsion 14. In this case, inorder to obtain the reference light beam, the light beam split by thebeam splitter 15 has its optical path changed by the mirror 16, the beamdiameter is magnified by the collimator lens system 17 and further, theoptical path is adjusted by the mirrors l8 and 19, so as to cause thelight beam to reach the photographic emulsion 14. Although the lens 9and the lens 1 1 need be disposed at common focal positions as shown inFIG. 3, the diffuser 10 need not be always arranged at a focal position.Rather, better results have been experimentally produced by disposingthe diffuser at positions other than a focus.

As described above, according to the present invention, the correlationof a required image is obtained with simple optical means, and the noisecomponent can be suppressed. Accordingly, with only slight modificationson hitherto-known hologram composing systems, the signal-to-noise ratioof reconstructed images can be enhanced without spoiling the redundancyof holograms.

We claim:

1. An apparatus for making Fourier transfer holograms comprising:

first means for providing first and second beams of coherent energy;

second means, disposed in the path of said first beam of energy, forconverting said first beam into a plurality of difiracted energy beamsand for spatially separating each of said plurality of diffracted beamsfrom the other of said plurality of diffracted beams;

third means, disposed in the path of said diffracted energy beams at alocation where said beams are spatially separated, for scattering saiddiffracted energy beams;

fourth means, disposed in the path of said scattered energy beams, forcollimating said scattered energy beams and directing said collimatedscattered beams upon an object; and

fifth means, disposed in the path of said second energy beam and thebeams directed toward said object, for forming a hologram of said objectwith a substantially increased signal-to-noise ratio.

2. An apparatus according to claim I, wherein said energy is light, saidsecond means comprises a transparent diffraction grating and a firstlens displaced with respect to said grating to receive said plurality ofsaid diffracted energy beams, and wherein said fourth means comprises afirst collimating lens, a focal point of which is coincident with afocal point of said first lens.

3. An apparatus according to claim 2, wherein said third means comprisesa diffuser disposed between said first lens and said first collimatinglens displaced with respect to the coincident focal points thereof.

4. An apparatus according to claim 3, wherein said second meanscomprises a first beam expander and a second collimating lens fordirecting an expanded collimated beam onto said diffraction grating, andfurther including a second beam expander and a third collimating lensdisposed in the path of said second beam.

5. An apparatus according to claim 2, wherein said third means comprisesa diffuser disposed between said first lens and said first collimatinglens at the coincident focal points thereof.

6. An apparatus according to claim 5, wherein said second meanscomprises a first beam expander and a second collimating lens fordirecting an expanded collimated beam onto said diffraction grating, andfurther including a second beam expander and a third collimating lensdisposed in the path of said second beam.

,4 any

1. An apparatus for making Fourier transform holograms comprising: firstmeans for providing first and second beams of coherent energy; secondmeans, disposed in the path of said first beam of energy, for convertingsaid first beam into a plurality of diffracted energy beams and forspatially separating each of said plurality of diffracted beams from theother of said plurality of diffracted beams; third means, disposed inthe path of said diffracted energy beams at a location where said beamsare spatially separated, for scattering said diffracted energy beams;fourth means, disposed in the path of said scattered energy beams, forcollimating said scattered energy beams and directing said collimatedscattered beams upon an object; and fifth means, disposed in the path ofsaid second energy beam and the beams directed toward said object, forforming a hologram of said object with a substantially increasedsignal-to-noise ratio.
 2. An apparatus according to claim 1, whereinsaid energy is light, said second means comprises a transparentdiffraction grating and a first lens displaced with respect to saidgrating to receive said plurality of said diffracted energy beams, andwherein said fourth means comprises a first collimating lens, a focalpoint of which is coincident with a focal point of said first lens. 3.An apparatus according to claim 2, wherein said third means comprises adiffuser disposed between said first lens and said first collimatinglens displaced with respect to the coincident focal points thereof. 4.An apparatus according to claim 3, wherein said second means comprises afirst beam expander and a second collimating lens for directing anexpanded collimated beam onto said diffraction grating, and furtherincluding a second beam expander and a third collimating lens disposedin the path of said second beam.
 5. An apparatus according to claim 2,wherein said third means comprises a diffuser disposed between saidfirst lens and said first collimating lens at the coincident focalpoints thereof.
 6. An apparatus according to claim 5, wherein saidsecond means comprises a first beam expander and a second collimatinglens for directing an expanded collimaTed beam onto said diffractiongrating, and further including a second beam expander and a thirdcollimating lens disposed in the path of said second beam.